1. Field of the Invention
The invention is directed to a technique for improving the kinematic performance of robotic manipulators. More particularly, this invention provides a novel technique to accurately and efficiently identify the actual kinematic parameters of any manipulator and thus improve and reduce the variability in the kinematic performance of robotic manipulators.
2. Description of the Prior Art
A primary objective in the control of robotic manipulators is the accurate positioning and orientation of the end effector and cartesian space. It is the task of the robot kinematic controller to transform the desired end-effector cartesian trajectories into equivalent joint trajectories. The joint trajectories are the reference inputs to the joint servo systems which drive the joints to the desired positions. Thus, the input to a kinematic controller is a series of desired end-effector cartesian positions and orientations and the output is a corresponding series of joint positions. To perform the transformation, the kinematic controller must evaluate the inverse kinematic equations for each discrete cartesian location in both position and orientation. The inverse kinematic equations are derived from a parametric model of the relationship between the robot end-effector cartesian location and the joint positions. This model is called the "forward kinematics" of the robot. The parameters of the forward kinematic model described the spatial relationship between the lengths of the robot and are defined via the principles of geometry.
It is the conventional process in the robotic industry, to use kinematic parameters in the design and implementation of robot controllers. These kinematic parameters are obtained from the mechanical drawings generated through manufacturing specifications of the robot. The advantage of this approach is that it leads to relatively simple, closed-form control algorithms. The disadvantage, however, is that this approach implicitly assumes that the errors incurred during the manufacture of a robot are negligible. The actual kinematic structure of a robot will, in part, be a function of these manufacturing errors. Since manufacturing errors are random, each manipulator possesses unique kinematics and the positioning performance of these robots degrades significantly when there is a mismatch between the actual robot and the model which implements the kinematic control. While the manufacturing errors may be small, i.e. on the order of thousandths of an inch and hundredths of a degree, they can have a significant impact upon kinematic manipulator performance. Clearly then, manipulator performance degrades as the mismatch between the ideal kinematic parameters used in the design of the controller and the actual robot kinematic parameters increases. This mismatch is especially prevalent in manipulators with revolute joints in which small manufacturing errors produce significant errors between the actual and predicted positions and orientations of the end effector. There has been a long-standing need in the industry to develop practical methods for identifying the actual kinematic models of n that's degree of freedom robotic manipulators in order to improve their kinematic performance. Such models, whose parameters are estimates of the unique kinematic parameters, are known as "arm signatures". Once identified, the arm signature of an individual robot can be used to synthesize and implement control algorithms to improve kinematic performance.
Kinematic parameter identification algorithms have been proposed in the literature. Attention is directed to the following articles:
Robot Positioning Accuracy Improvement Through Kinematic Parameter Identification, Chen, J. et al., Proceedings of the Third Canadian CAD/CAM in Robotics Conference, June 1984;
An Improved Method For Identifying The Kinematic Parameters In A Sixth Axis Robot, Mooring, B. W. et al., in Book Title, pages 79-84, 1985; and
Industrial Robot Calibration Methods And Results, Whitney, D. E. et al., in the Proceedings of the International Conference on Computers and Engineering, ASME, August 1984.
The contents of these articles is incorporated by reference as is fully set forth herein.
It is therefore an object of the present invention to provide a method for improving and reducing the variability in the kinetic performance of robotic manipulators.
It is also an object of this invention to provide a technique for the accurate and efficient identification of the actual kinematic parameters of an industrial manipulator. This manipulator can have either revolute or prismatic joints.